A known wire electrode member and apparatus for electrical discharge machining is described in U.S. Pat. No. 4,287,404 wherein the electrically conductive metal wire is provided with an active surface comprising at least 50% by weight of a metal or alloy selected from a group consisting of zinc, cadmium, tin, lead, antimony and bismuth. In the employment of said type machining electrode, the active surface coating is continuously replenished during the machining operation by passage through an electrolytic bath containing the selected active metal. Such continuous replenishment of the active surface while the workpiece is being machined understandably renders the equipment more complex to build and operate. Moreover, the replated wire electrode is said to further require resizing with a die prior to its being fed to the machining zone.
In the aforementioned pending U.S. patent application, an adherent carbon coating provides the active surface for said electrical discharge machining electrode to provide a higher metal removal rate from the workpiece without accompanying need to continuously replace the active surface material. Other benefits attributable to said carbon or graphite surface on the machining electrode include low friction with the wire guides enabling offset of the wire electrodes member for taper cuts and/or higher back tensions which is desirable for wire alignment without producing vibration. As further indicative of these benefits, a 0.004 inch diameter electrode having the carbon surface coating deposited on a molybdenum core permitted machining of tool steel at thicknesses up to 4 inches with only a minor reduction in the metal removal rate at increased workpiece thickness and no wire breakage.
It still remains desirable, however, to further improve the cutting action achieved with said carbon coated wire electrode members with respect to both cutting speed and surface finish.
A further desirable objective is to improve the cutting action for this type wire electrode without requiring that the machining process or apparatus utilize higher levels of electrical power to achieve the same degree of cutting action.
Other objectives of the present invention will be apparent in the following detailed description.